In recent years, a technique by which a thin film transistor (TFT) is manufactured using a semiconductor thin layer (with a thickness of about several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors to be used especially as switching elements in image display devices is being pushed. Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is needed for liquid crystal displays and the like.
Some metal oxides have semiconductor properties. The examples of such a metal oxide having semiconductor properties are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such a metal oxide having semiconductor properties is already known (Patent Documents 1 and 2).